In various image displays such as a liquid crystal display, a birefringent film generally is used for the purpose of, for example, solving coloring or widening a viewing angle, in order to improve display characteristics. In general, such a birefringent film is produced by stretching a polymer film uniaxially or biaxially so as to adjust the birefringence or retardation of the polymer film.
In the case where such a birefringent film is used in a liquid crystal display together with a polarizing film, it is generally necessary to arrange the birefringent film and the polarizing film so that the slow axis of the birefringent film is parallel to the transmission axis of the polarizing film. In general, the direction of the slow axis of a birefringent film coincides with the film stretching direction, while the direction of the transmission axis of a polarizing film coincides with the direction perpendicular to the film stretching direction.
However, arranging the birefringent film and the polarizing film in the above-described manner involves the following problem in production. That is, when a polymer film is stretched by an industrial process, the polymer film generally is moved in the longitudinal direction so as to be wound around a roller while being subjected to a stretching treatment. When the birefringent film and the polarizing film that have been respectively wound around rollers as described above are attached to each other, continuous attaching can be achieved by winding them again around a roller while attaching them to each other in the state where the longitudinal directions of the respective films are directed in the same direction. Accordingly, in order to arrange these films so that the transmission axis and the slow axis are parallel to each other and attach them continuously, it is necessary that the polarizing film is stretched in the longitudinal direction whereas the birefringent film is stretched in the width direction. That is to say, in order to allow the direction of the transmission axis of the polarizing film (i.e., the direction perpendicular to the stretching direction) and the direction of the slow axis of the birefringent film (i.e., the stretching direction) to be the width directions of the respective films, the birefringent film needs to be stretched in the width direction. However, if a polymer film is stretched in the width direction continuously to produce a birefringent film, a so-called bowing phenomenon may occur so that an alignment axis within the plane of the film has a fan-like shape, for example. Therefore, it is difficult to achieve uniformity in alignment axis, birefringence, and retardation by stretching the polymer film in the width direction.
On this account, as a method of stretching a polymer film in the width direction, there has been employed a method in which a polymer film that has been stretched in the width direction is then subjected to a relaxation treatment for causing the width thereof to return to the original width, thereby improving the uniformity in slow axis in the width direction (the stretching direction). However, since the polymer film generally is formed on a base directly and then is stretched together with the base, the method may be affected by variation in physical properties of the base. Moreover, depending on the type of the polymer film, sufficient relaxing effect cannot be obtained. Therefore, according to this method, it is difficult to produce uniform birefringent films stably by an industrial process. Furthermore, although there has been an attempt to suppress the bowing phenomenon by cooling the polymer film after stretching, this cannot avoid the bowing phenomenon completely (e.g., Non-patent Document 1). Also, there has been disclosed a method in which a base on which a polymer film is formed is held between chucks of a tenter stretching machine in a loosened manner and the base is then heat-shrunk (e.g., Patent Document 1). However, this method has problems that the base used is limited to a heat shrinkable base and that, if the base is too thick, wrinkles may be formed when loosening the base, thereby making it difficult to loosen the base stably. Other than the above-described methods, there have been disclosed methods in which conditions for carrying out film stretching are determined, such as, for example, a method in which the width of a stretched film is set to be the square root of a stretch ratio (e.g., Patent Document 2), a method in which the width by which a film shrinks due to stretching in the longitudinal direction is defined (e.g., Patent Document 3), and a method in which a film that has been stretched is subjected to thermal relaxation (e.g., Patent Document 4). However, from an industrial viewpoint, it is difficult to produce birefringent films stably according to these methods.                Non-patent Document 1: T. Yamada et al. Intn. Polym. Process., Vo. X, Issue 4, 334-340 (1995)        Patent Document 1: JP 6(1994)-51116 A        Patent Document 2: JP 3(1991)-23405 A        Patent Document 3: JP 2(1990)-191904 A        Patent Document 4: JP 5(1993)-249316 A        